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North Cascades Geology

Sea-Floor Spreading

spacer image Using their simple tools compass, hammer, hand lens, and microscope, geologists in the first half of the 20th century learned much about the continental crust, but little about the oceanic crust. This changed following World War II, when ocean-going geologists adapted sensitive magnetometers developed for antisubmarine warfare for use in seafloor research. This modern technology gave geologists their greatest boost in more than a century of field work.

Comparing magnetic stripes on the sea floor to the magnetic reversals in basalt on the land as seen in a vertical cliff
Comparing magnetic stripes on the sea floor to the magnetic reversals in basalt on the land as seen in a vertical cliff. Adapted from Raft and Mason (1961) and Tabor (1987).

spacer image Magnetometer readings revealed that rocks on the ocean floor were magnetized in a startling pattern of symmetrical stripes

Click on image to see animation.

The magnetic polarity of every other stripe matched the normal polarity of the earth today while the alternate stripes had a reversed polarity. This means that if you could isolate a normal stripe from earth’s magnetic field and hold a a compass needle close to it, the north-seeking pole of the needle would align with the weak magnet of the rock stripe and point in the same direction as the earth’s north magnetic pole, but if you put the same needle next to a reversed stripe, the north-seeking pole of the needle would point in the direction of today’s the south magnetic pole. The stripes were of different widths and ran parallel to oceanic ridges. Most remarkably, the pattern of wide and narrow stripes of alternating polarity that occurred on one side of a ridge were mirrored exactly on the other.
spacer image The reason for the pattern became known when geologists studying the ocean floor showed their data to geologists who had noticed a similar pattern of alternating polarity in thick piles of surface lava flows. Geologists also had obtained radiometric ages of the lava flows (For another example of radiometric dating, see the Field trip stop at Newhalem). They knew how many thousands or millions of years ago each lava flow had erupted. The pattern of long and short time intervals represented by the polarity changes in the lavas matched the pattern of the ocean floor stripes. The scientists concluded that the lavas on land and the ocean-floor basalt had both formed at times when the Earth alternated between normal and reversed magnetic fields. Furthermore, the oldest stripes were farthest from the ridges, suggesting that the floor was growing and spreading by addition of material at the ridges.

Click on image to see animation.

spacer image As the ocean floor spreads apart, new lithosphere is created. This lithosphere is basaltic crust formed from magma welling up along the widening crack between two crustal plates moving in opposite directions. The polarity of the earth’s magnetic field is recorded in this new crust; hence, the magnetic stripes. The newly-formed lithosphere is hot and thus less dense than older adjacent lithosphere; as a result it floats high on the mantle, forming the largest chains of mountains on Earth, the mid-ocean ridge systems. Since ocean-floor spreading continuously creates new lithosphere, and the Earth is not growing, existing lithosphere must somehow be destroyed. The question is where does the existing lithosphere go?

Earthquake locations beneath Japanese volcanic arc.
Earthquake locations beneath Japanese volcanic arc. (Land elevation in this cross section much exaggerated. After Benioff, 1954).

spacer image Geophysicists had long observed a concentration of earthquakes along and under volcanic arcs, such as in Japan and the Andes mountains. The sources of the earthquakes, plotted from data gathered over the years, cluster in planes that dip landward under the mountains. These planes could explain the missing oceanic lithosphere if they were gigantic faults between the disappearing ocean floor and overiding crust. And in fact, the earthquakes in these dipping planes (known as Benioff-Wadati zones after their discoverers) are generated as an ocean-floor plate disappears into the mantle under an adjacent plate in a process known as subduction. Some of the earthquakes, especially those that are shallow and near the ocean itself, can be very destructive. Many quakes originate within the overriding continental plate, and some very deep ones appear to originate in the descending plate, where it is breaking apart in the depths of the mantle.

Sketch of plate tectonics scheme and formation of the major rock types: igneous, sedimentary, and metamorphic.
Sketch of plate tectonics scheme and formation of the major rock types: igneous, sedimentary, and metamorphic.

spacer image Geologists now know that the earth’s crust is composed of many such tectonic plates, floating on the denser mantle. Some are being created and expanding by the injection of upwelling mantle, others are more passive and are moved to and fro by the movements of their neighbors. Much geologic action takes place at the boundaries of tectonic plates. Besides where the plates spread apart, where new lithosphere is made, and subduction zones, where lithosphere is destroyed, there are boundaries, world-class faults where lithospheric plates slide past one another. Such boundaries are called transform boundaries. Perhaps the best known transform boundary is the San Andreas fault in California, where the Pacific plate slides north past the North American plate at a speed of about 1 1/2 inches per year.

See more animations showing plates in action.
or
Dig deeper into plate tectonics
or
Continue to Rocks Form Where Plates Collide


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This page was last updated on 12/1/99


Material in this site has been adapted from a new book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle